Eutectic separation using an electric field



Jan. 9, 1968 w. G. PFANN ETAL 3,362,898

EUTECTIC SEPARATION USING AN ELECTRIC FIELD 3 Sheets-Sheet 1 OriginalFiled Feb.

COMPOSITION DISTANCE FIG. 2

FIG. 3C

DISTANCE INVENTORS W PFA/V/V R. S. WAGNER ATZORNEI Jan. 9, 1968 w. G.PFANN ETAL 3,362,898

EUTECTIC SEPARATION USING AN ELECTRIC FIELD 5 Sheets-Sheet 2 OriginalFiled Feb.

FIG. 4

United States Patent 3,362,898 EUTECTIC SEPARATEON USING AN ELECTRICHELD William G. Pfann, Far Hills, and Richard S. Wagner,

Basking Ridge, N.J., assignors to Bell Telephone Lahoratories,Incorporated, New York, N.Y., a corporation of New York Continuation ofapplication Ser. No. 170,457, Feb. 1, 1962. This application Nov. 3,I966, Ser. No. 591,922 3 Claims. (Cl. 204-180) ABSTRACT OF THEDISCLOSURE Minimum melting point solutions such as eutectics areseparated under the influence of a D-C electric field while maintainingthe solution at a temperature intermediate that of the melting point ofthe total composition and a desired end product.

This application is a continuation of application Ser. No. 170,457,filed Feb. 1, 1962, now abandoned.

This invention relates to the separation of the components of eutecticand other minimum melting point compositions. The inventive processesare applicable to the separation of elements such as metals andsemiconductors, as well as compounds such as oxides and salts, and alsoto combined systems containing one or more components from each of theforegoing classes.

In general, the processes herein may be utilized in the separation ofany type of system providing (1) it is capable of undergoing a phasetransformation from liquid to solid, and (2) that the components have acharacteristic difference of a type which will permit a redistribution,no matter how slight, by the application of a unidirectional fieldacross the liquid. While in the preferred embodiment the external fieldtakes the form of an electrical field and the characteristic differencein the components takes the form of a differential ionic mobility, othertypes of fields, suitably applied to take advantage of differentcomponent characteristics, are set forth herein. One other such fielddiscussed specifically, and to which an example is directed, makes useof a thermal gradient. Other fields include gravitational, magnetic, etcetera.

While it is recognized that all of these influences, together withresulting redistributions, have been observed in systems of the classeshere under consideration, these effects have thus far been of scientificinterest only, largely by virtue of the fact that the redistributions soeffected are generally small and serve only to illustrate the principlesinvolved.

These separation mechanisms, however, attain practical significance whenmeans is provided for progressively removing the redistributedcomponents with continuous application of the field. It is common to allof the processes herein that the progressive removal of such componentsresults from the maintenance of a temperature here designated T at atleast one position within the liquid body correspondin with a point atwhich the distributing effect is such as to result in a compositionhaving a liquidus temperature equal to or higher than T. It will be seenthat, by this expedient, distributed components are continuouslyremoved, often resulting in a steepening of the gradient produced by thefield along the decreasing path length defining the remaining liquidbetween two such freezing interfaces.

While the invention is described largely in terms of the separation ofsuch a eutectic or minimum-point mixture, it is to be recognized thatthe mixture need not have an initial composition exactly correspondingwith such minimum point. It will be shown that an advantage in 3,362,898Patented Jan. 9, 1968 separation accrues from operation on compositionshaving a liquidus temperature substantially higher than the minimumpoint.

'It is convenient to discuss the invention and the broad range ofsuitable systems to which the invention is applicable in uniform terms.For these purposes the term eutectic is to be understood as includingminimumpoint mixtures in general, of course assuming increasing liquidustemperatures as the composition deviates from the minimum point. Anon-eutectic minimum-point mixture is exemplified in the Nb-Zr system.The term components is intended to include the ingredients of any sucheutectic system, whether they be elements or compounds. As has alreadybeen mentioned, the inventive processes require the application of afield which, by its nature, has a redistribution effect on thecomponents of the system being separated. Hence, while the field soapplied may be electrical, thermal, gravitational, et cetera, it must beof such nature as to result in a composition gradient within the liquidbody to which it is applied. While persons skilled in the art are ingeneral familiar with the magnitude of eifects that may be obtained bythe application of specific fields to specific systems, it is seen thatto be useful for the purposes herein the resulting redistribution mustbe sufiicient to produce a compositional change such that at at leastone position in the liquid a composition having a liquidus temperaturehigher than some practical value of T is obtained. For these purposes, aT difitering from the eutectic or low melting temperature, T by 1 C. isconsidered adequate;

In summation, component separation of a liquid-phase eutectic system isschieved by the application of a field capable of producing aredistribution in the liquid sufficient to result in a local compositionhaving a liquidus temperature at least equal to a temperature T,representing a value at least 1 C. higher than the eutectic or minimumtemperature T, while maintaining the liquid at the said temperature T atleast in the region corresponding with the attainment of such liquiduscomposition.

Thedescription is facilitated by reference to the drawing, in which:

FIG. 1, on coordinates of temperature and composition, is a binarydiagram for a system having a classical eutectic compositional point;

FIG. 2, on the same coordinates, is a binary diagram of a system havinga compositional point manifesting a minimum value of liquidustemperature, deviation from which compositional point in eitherdirection results in an increase in liquidus temperature;

FIG. 3A is a schematic cross-sectional view of one type of apparatussuitable for the practice of the present invention;

FIG. 3B, on coordinates of component concentration and distance, is acorresponding plot of the concentration gradient resulting in theapparatus of FIG. 3A;

FIG. BC, on coordinates of liquidus temperature and distance, is a plotshowing the liquidus temperature variation along the gradient producedby the application of the field in the apparatus of FIG. 3A;

FIG. 4 is a front elevational view, in section, of an alternate form ofapparatus suitable for the practice of the inventive process herein,dilfering from the apparatus of FIG. 3A in the provision of reservoirs;

FIG. 5 is an elevational view, partly in section, of yet another type ofapparatus particularly suitable for operation on a particular class ofcompositions;

FIG. 6 is an elevational view, partly in section, of apparatus suitablefor the separation of compound systerns;

FIG. 7 is a schematic view of a Clusius column intended for theseparation of eutectic systems by application of a thermal field; and

FIG. 8 is a front elevational View, partly in section, of an alternateform of apparatus suitable for the separation of components under theinfluence of a thermal field.

In large part, the detailed description of the invention, including manyof the figures, is in terms of the use of an electrical field. Adescription in these terms is not only expedient but is particularlysuitable in that the preferred embodiment herein makes use of such anelectrical gradient. However, description in these terms is not to beconsidered as limiting the scope of the invention, it being recognizedthat the principles discussed apply equally well to the application offields of the other types discussed herein, many of which are suitablyapplied in the apparatus depicted.

Referring now to FIG. 1, the binary diagram presented is that of asimple eutectic system of two components A and B. The designations T andC refer respectively to eutectic temperature and eutectic composition.The symbol T has already been discussed.

FIG. 2 is a binary diagram of a simple two-component system, againreferred to as A and B, however manifesting a minimum liquidustemperature and composition for expediency also designated T and Crespectively, the symbol T again representing a temperature intermediateT and the melting point of at least one of components A and B. It isrecognized that components of compositions C of either of the systemsshown on FEGS. 1 and 2 are not readily amenable to separation byphysical means. The two systems for which binary diagrams are presentedexemplify materials to which the invention is applicable. Of course,eutectic compositions occur in systems having more complex compositionaldiagrams, either with or without partial liquid-solid solubility. Suchsystems include ternary or higher order mixtures of elements orcompounds, as well as higher order systems containing one or morecomponents of either class. The processes and apparatus herein areequally applicable to all such systems.

At this point in the description of the invention, it is convenient torefer to the simple apparatus depicted in FIG. 3A. While this apparatusmay be applied efliciently to the separation of conductive elementalsystems, it will be seen that one or other of the alternate formsdepicted in subsequent figures may more expediently be applied to theseparation of systems showing proportionality as compared withelectronic conductivity. The principles here discussed are, however,applicable to all systems upon which the invention may be practiced.

The apparatus of FIG. 3A consists simply of a capil lary tube 1 of abore of 1 or 2 millimeters filled with liquid metal of the eutecticcomposition C and having sealed inert electrodes 2 and 3, as shown. Inpractice, provision must generally be made for expansion or contractionduring processing. Such provision, which may take the form of slidingjoints, flexible members, or simply a portion of unfilled tube is, forsimplicity, omitted. Redistribution is effected by producing aunidirectional electrical field of at least 0.02 volt/cm., which maypreferably be of the order of from about 0.1 to 1.0 volt/cm. length inthe direction of field, across the liquid by the passage of directcurrent between the electrodes 2 and 3 by means of current source, notshown. Since the conductivity of liquid metals is of the order of 10ohmcm.- the current density is typically of the order of about 10 amp/cm. or higher.

For the exemplary binary metallic system treated, the currentresponsible for the field is carried almost entirely by electrons.However, metallic ions have a small but definite mobility which may beas high as of the order of 10- cm. volt-sec. Any such cations tend tomove toward the negative electrode under the influence of the appliedfield. For the system under consideration, however, two metallic cationspecies are present, it being assumed that their differential mobilityis of the order of at least 3X10 cm. sec. for the field applied, andthis is a general requirement for components of systems which are to beseparated in accordance with the instant invention by the use of anelectrical field. Although both cation species have a tendency to movein the direction of the negative electrode 3, that species having thegreater mobiiity, here assumed to be the B ions, displaces the slower Aions from the vicinity of the negative cathode. In time, assuming theabsence of convection and a sufficiently high temperature to preventfreezing, a steady state continuous gradient of concentration withmaximum B ion concentratlon in the vicinity of the negative cathode isestablished along the tube. Under certain simplifying assumptions, thisgradient is exponential in form. The steady state condition owes itsexistence to the limitation of movement of B ions under the influence ofthe applied field by back diffusion.

The situation discussed in the preceding paragraph obtains only underthe temperature condition noted, that is, a temperature throughout thetube at all times greater than any liquidns temperature which mayobtain. To accomplish the continuing separation upon which the inventiveprocesses depend, the liquid within tube 1 is at least locallymaintained at a temperature T which is above the eutectic temperature Tbut which is below the liquidus temperature of some compositionresulting from the redistribution effected by the applied field, atleast in the vicinity in which this composition is produced. In asimplified case, which finds practical application where separation isbrought about under the influence or" an electrical field, it isconvenient to maintain the entire body of liquid within the tube at suchtemperature T which may be chosen 5 or 10 degrees above T as shown inFIG. 1. Under these conditions, the solute distribution at an earlystage of the process (before freezing) is as represented in FIG. 3B, andthe corresponding liquidus temperatures are as shown in FIG. 3C. For theconditions shown, the exponential distribution brought about by theapplication of field has resulted in the attainment of a B-richcomposition in the vicinity of the negative electrode having a liquidustemperature equal to T. This results in the freezing of an increment ofthe liquid at the cathode and a consequent enrichment of the remainingliquid with respect to A. This, in turn, under the influence of thefield, results in a steepening of the gradient in the vicinity of theanode, so in time resulting in the attainment of a liquidus temperatureT and the commencement of freezing at that electrode.

From this time on, the rates of growth of a and B solid solutions aredetermined by the fluxes of A and B ions arriving at the freezinginterfaces, which latter are, in turn, direct functions of the field andof the differential mobility. Eventually, the entire body of liquidfreezes, forming two sections, one of a, the other of ,8, and theeutectic mixture is separated into its component phases.

It is evident at this point that the compositions of the a and 5 phasesare deter-mined by T or, more generally, by the temperature ortemperatures at which the freezing interfaces are maintained. It isclear that increasing the magnitude of T results in more completeseparation of the eutectic system into the components A and B, withultimate distribution being obtained for those conditions under whichthese temperatures correspond with the liquidus temperature for the mosthighly enriched a and ,8 phases which may be obtained under theparticular field and back diffusion conditions chosen. In general, sincemore economical and highly efiicient separation procedures are availablefor the processing of non-eutectic mixtures, it appears desirabie to setT at a minimum value, for example of the order of as little as 1C. abovethe eutectic temperature, for carrying out further purification of u orB by use of zone melting or other well-known procedure.

Certain simplifying assumptions have been made. For example, while formost metals the electrical and thermal conductivities are greater in thesolid than in the liquid phase, so resulting in stable junctions, thereverse is generally true of semiconductive systems. Interfaces may bestabilized by the use of external temperature fields or the problem canbe avoided by freezing outside of the electrical path, so avoiding thepassage of current through the liquid-solid interface (see FIG. 5 andrelated discussion).

The apparatus of FIG. 3A utilizes a liquid column of small bore. This isparticularly advantageous for metallic systems since the Joule heatingresulting from the large electronic fiow required to produce therequisite field, in turn, demands either across section of large surfaceto area ratio or forced cooling. The small bore is useful, too, inminimizing convective mixing. If the liquids at the ends of the columndiffer in density, mixing may be further reduced by inclination of thetube.

The advantages which accrue from use of a capillary tube are retained inthe apparatus of FIG. 4, which is at the same time provided withreservoirs and 11, which, where separation is to be brought about by anelectrical field, may be made of graphite or other conductive material.Capillary tube and electrodes are designated 12, 13, and 14,respectively. While reservoirs 10 and 11 are shown as having equalvolumes, these are desirably proportional to the volumes of 0c and [3 inthe eutectic. The relative levels of the capillary tube 12 and thereservoirs 10 and 11 are determined by the freezing pattern, with a viewto preventing blocking of the how of liquid into the tube beforefreezing is complete. The freezing pattern is, in turn, determined bythe conductivity of the reservoir walls and the relative densities ofthe solid and liquid phases. While it would appear that the capillarytubes of both the apparatus of FIGS. 3A and 4 should be insulating, andthis is generally desirable, the usually attendant .low thermalconductivity is undesirable from the standpoint of heat removal. Incertain instances it may be feasible to use a thin-wall tube ofstainless steel or other conductive material which, while it contributesto Joule heating, nevertheless permits the passage of larger currentsdue to its lower thermal impedance. This choice, as between conductingand non-conducting walls, as well as other design considerations, isfacilitated by studies set forth in the literature (see, for example,Elektrolytische Wanderung in flussigen und festen Metallen, K. E.Schwarz, J'. A. Barth, Leipzig (1940)).

It has been noted that problems attendant on comparatively lowelectrical conductivities in the solid phase may be avoided by the useof an apparatus arrangement providing for freezing of a and p phases onsurfaces out of the current path. Such arrangements, useful also in thetreatment of fused salts manifesting low conductivity in the solid, areexemplified by the apparatus of FIG. 5. The apparatus differs from thatdiscussed previously 3t? and 31 separated by a porous insulating barrier32 and provided with anode and cathode 33 and 34, respectively. Theapparatus differs from that discussed previously in that the bulk liquidis maintained at a temperature higher than that corresponding with theliquidus temperature of the most highly enriched, highest melting pointcomposition resulting, and provides instead for the freezing of 0c and{3 crystals on internally cooled members 35 and 36 which, in the mostsimple instance, are both maintained at a temperature T. The apparatusis depicted at an intermediate phase in processing and so shows initiallayers of frozen ct-phase material 37 and fi-phase material 38.

Discussion has thus far been in terms of the treatment of systems inwhich conduction is, in large part, elecronic. Systems of this type, inwhich the electrical field is produced in large part by electronic flow,are most convenient from a pedagogical standpoint in distinguishingbetween the use of electrical fields here under discussion andconventional processes of electrolysis. The functions of current fiowresulting in the field and ionic separation are recognized as distinct.The ions which do not carry current migrate in opposite directions andare so separated. The electrons, which do carry the current, merelyserve as the means of establishing the electrical current in the liquidthat causes the migration. However, while more involved considerationsmust be taken into account, the inventive procedures herein may operatewith equal or even greater facility on eutectic mixtures of salts oroxides. Advantages in the treatment of such materials, as compared withconductive metals, accrue from the fact that larger fields may beproduced for a given amount of Joule heating, so resulting in more rapidseparation. This follows from the fact that ionic mobilities, andconsequently the expected order of differential mobilities for suchionic materials, are of the same general order as for metals. On theother hand, such materials have the disadvantage that, to the extenttheir conduction is ionic, electrode reactions may have to beconsidered. Also, as already noted, electrical conductivity in an ionicconductor is often much smaller in the solid than in the liquid, so thatsolid is usually not permitted to block the current.

The field freezing of a fused salt or oxide mixture of the type AB-AC,where A denotes the common metallic cation and B and C, differentanions, or of an AC-BC (common anion) system, may be carried out on theapparatus of FIG. 6. The apparatus there depicted consists of reservoirs4t and 41, which again may be constructed of graphite or otherconductive material. Where the system under treatment is of the typeAB-AC, each graphite crucible additionally contains an electrode ofmetallic A (42 and 43), here shown as if molten, as well as eutecticliquid 44 to be separated. Current assumed to be solely ionic is carriedentirely by A ions which form at the anode surface 45, migrate throughthe capillary 46, thereby setting up an electric field, and are reducedto metal at the cathode surface 47. Meanwhile, anions B and C, assumedto differ in mobility, migrate in opposite directions but take no partin the fiow of current. Internally cooled members 4-8 and 49 aremaintained at temperature T, as hereinbefore defined. When the liquidustemperatures in the reservoirs rise to T, aand ,8- phase materials 59and 51 freeze out as shown. At the end of the operation, essentially allof the liquid 44 will have solidified as a or B and metal A will havebeen transferred in the negative reservoir in amount proportional to theproduct of current and time.

The above AB-AC system is a special use of the ABC!) system (A and Cboth metals). AB-CD mixtures may be separated as discussed withelectrodes 42 and 43 containing the metal which deposits cathodically atthe lower voltage.

To separate a eutectic of the type AC-BC by field freezing, where C may,for example, be chloride, a chlorine cathode may be used with provisionfor recirculation of chlorine gas evolved at the anode back to thecathode.

These considerations can be extended to more complex systems, forexample, ternary liquids in which the third component might be water, ora metal soluble in either of the ionic compounds. In the latter case theelectrons of the molten metal might carry all the current with none ofthe other ions involved. In that event, electrode reactions would beavoided and the separation of u and B could be performed as describedfor metallic liquids.

It is evident that, while electric field separation is generallyconsidered the preferred mode of operation, the progressive freezingtechnique described herein may be applied to mixtures in whichseparation is effected by other means, either alone or in combinationwith the use of an electric field. It has been noted in the past byworkers in the art that separation may be brought about by use ofgravitational fields and magnetic fields, as well as thermal fields.While the first two are of limited utility and perhaps applicable toonly a minority of systems encountered, the use of a thermal field orgradient has a most universal utility.

A two or more component liquid solution placed in a thermal gradientdevelops a compositional gradient. For thermal gradients of the order offrom 10 C. to 100 C./c1n., the composition diiference is usually small,although it may be of the order of several percent. It the operation iscarried out in a Clusius column, the countercurrcnt flows of hot andcold liquid can greatly enhance the compositional gradient. Such anapparatus is depicted schematically in FIG. 7. The most simple form ofClusius column, as depicted, consists of a vertical enclosure 60terminating at either end in enlarged portions 61 and 62. The apparatusis filled with eutectic solution, again with provision, not shown, forexpansion or contraction brought about by freezing, and one wall of therestricted enclosure, such as right-hand wall 63, is heated or ismaintained at a temperature higher than that of opposite, or left-handwall 64, the temperatures of both of walls 63 and 6d, however, beingmaintained above the freezing point of the most highly enriched phaseproduced in the system. Distribution of components due to the thermalgradient between walls 63 and 64, resulting in an assumed migration of Ato a hot wall 63, is accentuated by the countercurrent flows indicatedby arrows 65 and 66, so resulting in an A-rich a phase in the upperenlarged portion 61 and a B-rich 6 phase in the enlarged portion 62.Maintenance of cooling coils 67 and 63 at different temperatures ashereinbefore described results in separation of the melt into separatecrystalline fractions of a on 67 and [3 on 68.

The operation carried out in the apparatus of FIG. 7 has been describedin fundamental terms. It is evident that this operation can be carriedout on more complex systems including any of those discussed previously,and that further separation may be produced by feeding either the a orthe B phase into the same or a similar piece of apparatus, melting, andrepeating the operation.

The separation of walls 63 and 64 of the Clusius column may range fromof the order of 0.025 to the order of 0.10 cm., typically being about0.05 cm, while the height of the column may range from one to severalfeet or more. The essential features of the thermal diffusion methodhave been described. Many variations of the ap paratus can be envisaged.A different apparatus is depicted in FIG. 8. This apparatus consists ofreceptacles 70 and 71, partially separated by walls '72 and 73, whichare in turn set a distance of from 0.025 to 0.10 cm. apart. Theprinciple of the Clusius column is retained by passing coolant throughpath '74 in wall '72 and by means of heater 75 in wall 73.Countercurrent flows accentuate the separation in the gradient soproduced in the manner described in conjunction with FIG. 7, with a and,8 materials 76 and 77 freezing out on the surfaces of internally cooledfreezing members 78 and 79, respectively. More eflicient operation isassured by use of stirring means such as 80 and 81.

Again, the technical literature is helpful in supplying information asto suitable gradients and flow rates for various systems (see, forexample, Techniques of Organic Chemistry, vol. 3, part I, 2d ed. 1956,Interscience Publishers, New York).

The following examples illustrate systems amenable to processing by theinventive procedures.

Exam ple 1.Bismuth-tin eutectic A eutectic alloy of 4-3 atomic percentbismuth is melted in an evacuated glass tube and allowed to flow througha constriction in the tube (so as to remove oxide skin) into a glasstube of bore area 2.2 mm?, and cm. long, with molybdenum electrodes ateither end. The tube is maintained above the melting point of theeutectic, 144 C., so as to prevent freezing and possible cracking.

The tube is placed in an oven whose temperature is controlled by athermocouple in close thermal contact with the column of metallic alloy.The Joule heat of the current, the cooling provided by a flow ofnitrogen, together with the ambient heat furnished by the oven maintainthe liquid at about C. The current of 450 amp/crn corresponding to afield of about 0.1 volt/cm, causes tin to migrate to the negativeelectrode and hismuth to the positive electrode. Tin-rich on solidsolution freezes at the negative electrode and bismuth rich 5 solidsolution freezes at the positive electrode progressively inward from theelectrodes, until the entire liquid is frozen.

Example 2.-Gold-germanium culeclic Molten gold-germanium eutectic ispoured into an apparatus like that of FIG. 6, while at a temperaturesomewhat above 356 C. to prevent cracking of the glass capillary byunintentional freezing. The volumes of metal in the reservoirs arearound 2 cubic cm. each, but in the relative proportions of about 2.1 to1 in the gold (positive) and germanium (negative) compartments, respcctively. The glass capillary was 20 cm. long and 2.2 mm. in bore area.The current density was 820 amp/cm. Gold containing some germanium insolid solution froze in the positive graphite compartment, whileessentially goldfree germanium froze in the negative compartment. Theextreme Walls of the compartments were maintained at T:366 C. byconduction of heat by heavy copper electrodes extending outside the hotzone of the furnace. A flowing nitrogen atmosphere was used.

Example 3.NaF-Pbl eutectic Apparatus like that of FIG. 5 is filled withNaF-PbF eutectic, containing about 67 mol percent of PbF and placed in afurnace controlled to maintain the liquids in the compartments at about575 C. Internally gas cooled graphite freezing members, electricallyneutral, are maintained in each liquid at a temperature of about 565 C.Molten lead pools, resting at the bottom of each graphite compartmentserve as anode and cathode. A potential difference of 1.5 volts isapplied across the porous barrier, which may be of fritted silica ofabout 0.5 cm. thickness. Passage of current, by means of lead ionsmigrating from anode to cathode causes sodium ions to concentrate in thecathode compartment and lead ions in the anode compartment until theyreach such concentration as to freeze out as NaF and PbF on the freezingmembers. During the process there is a net migration of molten lead fromanode compartment to cathode compartment which occurs.

Example 4.MoO -WO A fused silica tube in apparatus like that of FIG. 3Ais filled with a molten eutectic containing about 2.5 mol percent of W0at a temperature above about 789 C. A field of about 2 volts per cm. isapplied between molybdenum electrodes at the ends of the tube, which isof about 2 mm. bore. The tube is externally flushed with nitrogen suchthat a substantially uniform temperature T '=780 C. is maintained in themolten electrolyte by the Joule heat of the current. M00 concentrates atthe negative electrode and freezes out, while W0 does the same at thepositive electrode.

Example 5 This example pertains to the use of thermal diffusion inseparation of lead-tin (38.1 weight percent lead), using the apparatusof FIG. 8. Members 78 and 81 are maintained at a temperature (T) of C.(2 above the eutectic temperature). The temperature of hot wall 73 ismaintained at 200 C. and that of cold wall '72 at C. The slit width orseparation between the hot and cold walls is 0.020 inch. Lead migratesto the cooler wall and lead-rich liquid descends to the lower reservoir.Lead-rich solid solution crystals freeze out on freezing member 78.Similarly, tina'ich liquid rises along hot wall '73 and tin-rich solidsolution crystals freeze out on member 81.

In general, it may be stated that eutectic components capable of beingredistributed by the application of a field so resulting in at least onelocalized composition having a liquidus temperature of the order of 1higher than the eutectic temperature, T may be separated by theprocesses herein. While it is inexpedient to set forth all known systemscapable of being so processed, persons skilled in the art will recognizethat comparatively few, if any, systems will defy processing. It hasbeen stated that the preferred embodiment herein is the use of anelectric field, either alone or in conjunction with an additionalinfluence. Several eutectics which may be separated by use of anelectric field are set forth below. Eutectic compositions andtemperatures are also indicated.

Metallic binary systems.-Bi-Sn, Au-Ge, Ag-Al, Bi-Cd, Bi-Pb, Cd-Pb,Cd-Sn, Pb-Sn, Pb-Zn.

Salts.NaF-PbF NaF-AIF NaF-CdF LiCl-KCl, CdBr -ZnBr LiCl-BaCl KF-LiF,NaF-LiF, CaF -NaF.

OXld2S.-NHF PbFg, N320 Na O-MoO C210, FeOTiO fAgO-Tlc V205'Pb0,Slog-T1102, MOO -WO Minimal melting point solid sluti0n.LiF-'\ IgF K-Cs,Rb-Cs, Ti-V, Mn-Ni.

The invention has been described in terms of a limited number ofembodiments. Variations on designs discussed have been proposed. Othersare apparent. The essence of the invention is the removal ofnon-eutectic compositions for simplicity designated a and 3 (even thoughfor certain systems this terminology is unconventional) by progressivefreezing. Regardless of the influence resulting in initialredistribution (here designated as field), be it electrical,gravitational, thermal, magnetic, et cetera, removal is effected bymaintenance of at least a portion of the liquid at a temperature abovethe eutectic temperature T but equal to or below the liquidustemperature of an enriched composition resulting by use of theseparating influence or field. Further, while initial separation may becarried out in a material which is off eutectic stoichiometry, eventual(or initial) separation of the eutectic in a progressive mannergenerally requires removal of material of two phases simultaneously. Ithas been assumed, for simplicity, in the description of certain of theembodiments herein that the temperature resulting in freezing of bothphases is equal to some temperature T. It is, however, evident, and ithas been noted, that certain conditions may dictate the use of differenttemperatures at the two freezing interfaces. It has been noted that Tshould be at least one degree above the eutectic temperature T Themaximum effective temperature of an interface corresponds with theliquidus temperature of the most highly enriched phase that can beproduced by the influence applied at that interface, with an absolutemaximum equal to the freezing point of a pure component attraced to thatinterface. It is seen that the temperatures of the two interfaces maytherefore differ and may have different maximum values, however, whilesharing the same minimum, above noted.

In certain instances, notably where the eutectic composition is close tothat of one of the components, it may be desirable to freeze out onlyone phase, which will usually be that corresponding to the highermelting component, from a liquid originally of eutectic or neareutecticcomposition. This may be for the purpose of securing a more perfectcrystal of that phase, or one free from even small amounts of theopposite phase. In this event, field-freezing will change the netcomposition of the remaining liquid in a direction away from thecomposition of the freezing phase, and this will occur if T is above thefreezing point of the pure component nearer which the eutecticcomposition lies. The process normally will end when the composition ofthe liquid in the vicinity of the freezing phase attains a liquidustemperature which is below the T maintained at the freezing surface.

The use of a constant temperature across the entire liquid body, whilepractical where the influence takes the form of an electric field in theseparation of a metallic system or other material evidencing reasonablesolid state conductivity, may be impractical as noted where solid stateconductivity is low. In such instances, it has been noted that freezingmay desirably be carried out at an interface removed from the electricalpath. Where the separating influence takes the form of a thermalgradient, this separating means inherently prohibits the use of aconstant temperature T across the entire body of liquid. Where it isdesired to maintain a constant temperature T at both the freezinginterfaces in a system undergoing phase transformation by use of athermal gradient, this may be accomplished by separating the freezinginterface corresponding with the low end of the gradient from the mainapparatus by a flow path through which liquids are caused to move bymeans of convection or a pumping means. Other alternative techniques areapparent.

What is claimed is:

1. Process for redistributing at least two components of a system havinga composition approximately corresponding with a minimum meltingcomposition of the said system comprising imposing a D-C electric fieldbetween two spaced conducting surfaces immersed in a liquid body of thesaid system, the said field being such as to result in the concentrationof at least one component of the said system to the extent necessary toproduce a local liquid composition having a freezing point at least 1 C.higher than the temperature corresponding with the freezing point ofsaid minimum melting composition, While maintaining a temperature havinga maximum value equal to the freezing point of the said localcomposition in a position in the liquid body approximately correspondingwith said concentration, said position being spaced from either of saidconducting surfaces.

2. Process of claim 1 in which the applied field is at least 0.02volt/cm. and in which the differential mobility of ions of the said atleast two components is at least 3 X10 crnP/sec. under the said appliedfield.

3. Process of claim 2 in which the said system is a eutectic system.

4. Process of claim 2 in which system at least one of the components isa salt.

5. Process of claim 2 in which at least two of the said components aresalts and in which application of the said electric field results inionization of the said components to produce a common ion.

6. Process of claim 5 in which the said conducting surfaces consistessentially of the element corresponding with said common ion.

7. Process of claim 2 in which at least one of the components is anoxide.

8. Process of claim 2 in which the said system is a noneutectic, minimalmelting point composition.

References Cited UNITED STATES PATENTS 6/1955 Rothstein 204- OTHERREFERENCES Dralrin: Izv. Sektora Fiz.-Khim. Analiza, Inst. Obshch.Neorgan. Khim., Akad. Nauk SSSR, 1950, vol. 20, pp. 341 to 344.

